Observer eye height (EH)-the point at which one's line of gaze intersects an object or scene-is a readily accessible source of information that can be used to scale the sizes of objects in the environment. In principle, this is accomplished by forming a ration of the objects's total height with the portion of it that falls below the intersection with observer EH. The purpose of this research is to investigate factors governing the utility of EH in perceiving size. Three main factors will be addressed. The first is the size of the object relative to the observer. There is obviously an optimal range of object sizes for which observer EH is used: the proposed experiments will delimit this range. The second factor is the posture of the observer. IF EH scaling is a common strategy for gauging object size, then it out be accessible from different postures, such as sitting. The proposed experiments will determine whether and how EH is used in different postural contexts. The third area of study is environment constraints; specifically, the assumption of common level ground between observer and object. Perturbing this relationship by having subjects judge object size on a slope (i.e., standing on a hill) will establish the efficacy of EH scaling independent of the natural horizon. In most studies, the use of EH will be explicitly tested by manipulating it unbeknownst to subject. This will be accomplished in two ways: 1)introduction of a false into the field of view; 2) the use of Virtual Reality (VR). In all experiments, within-subjects size judgements of rectangular objects in EH-manipulated and unmanipulated conditions will be compared.